Porous materials may be obtained by molding and sintering powders containing fibrous, dentritic, or spherical shaped precursor particles. The precursor particles are commonly metals, like platinum, or nickel, or their alloys, ceramic materials like alumina, or polymeric materials like polytetrafluoroethylene. In such porous materials, the strength of the material, the size of the pores in the material, and the surface area of the material are related to the packing density, the size, the shape, and the composition of the particles making up the powder. Sintering process conditions also affect the strength, pore size, and surface area of such porous materials. To achieve small pores and high surface area, the sintering of small diameter particles is preferred.
In materials with large pore sizes, the size of the pores may be further reduced using a variety of techniques. For some materials it may be possible to vapor deposit, electroplate, or electroless plate additional material into the pores of the base porous material. These methods result in uniform coverage and reduced pore size, but they also results in reduced surface area of the material. Alternatively, a slurry of particles is formed and applied by spraying or brushing the slurry onto the surface of the material and then sintering it after drying. This method does not ensure penetration of the particles into the substrate so as to occupy at least a portion of the inner pores. This method results in poor adhesion between the applied slurry and the porous substrate due to differential shrinkage of the slurry powder and the substrate surface during sintering. Further, this method may not build up of a layer or powdered precursor capable of sintering to form a porous structure.
Porous and high surface area materials are used in, catalysis, gas sensing, and filtration. For example, finely divided noble metals or alloys (Pd, Pt, and Rh) deposited on a porous ceramic or metal substrate may be used as a combustion catalyst to thermally decompose hydrocarbons gases; these types of catalysts may also be used to remove NOx and CO from exhaust gases. Porous materials may be used as electrodes in fuel cells where the electrolyte in the cell is a solid polymer. For proper operation, the polymer electrolyte in these fuel cells needs to be maintained in a hydrated form to prevent loss of ionic conduction through the electrolyte. In order to maintain membrane hydration and suitable reactions at the cell electrodes, one or more of the cell's electrodes may be made from very small metal particles (usually 2-5 nm diameter) that are distributed on, and supported by, larger conducting particles. These supported metal particles are formed into high surface area electrodes that are porous in order to optimize contact between the reactant gas, the electrolyte, and the metal catalyst. Pellistors are gas sensors having a porous metal electrode on a ceramic, (i.e. a ThO2 and Al2O3 ceramic pellet coated with a porous catalytic metal like Pd or Pt), that reacts with flammable gases to generate heat which is detected by an RTD embedded in the ceramic pellet. The detection limit for these sensors is related to the amount of heat generated by the decomposition reaction; this depends on the active area of the porous metal electrode.
Sintered ceramic and metal gas filters typically have pore sizes in the 1-10 um range and can remove particles down to 0.003 micron with a log retention value of greater than 9. In gases, particle capture is by diffusion and interception with the filter surface. Because of the low viscosity of gases, the filters are able to flow large volumes of gas with nominal pressure drop across the membrane. In liquids these same filters would only remove particles in the 1-10 um range with an LRV of about 2 because sieving is the dominant mechanism for particle removal or capture in liquids. Because of the higher viscosity of the liquid, the pressure drop across the same filters for a given volumetric flow rate would be greater for a liquid than for a gas. Supercritical fluids, those materials whose temperature and pressure are above the critical values, have properties that are intermediate between those of gases and liquids. Supercritical fluids generally retain the solvation properties and densities of the liquid while having gas like viscosities and surface tensions. Because of a supercritical fluid's solvating properties, it interacts with both the particle and filter surfaces, particles in supercritical fluids are preferably removed from the fluid by sieving rather than diffusion and interception. Because of their gas like viscosity and surface tension, supercritical fluids will have a pressure drops across the same filter more like a gas than a liquid. It is possible that smaller pores, nanometer sized pores or smaller, may be designed into the filter to capture nanometer and sub-nanometer size particles by sieving but without greatly increasing the pressure drop of the filter.
It would be desirable to have a mechanically strong, high surface area, material with small pores. Further, it would be desirable to be able to make objects with these properties with different materials and in a variety of shapes and sizes.